I had to make a warning on this topic.
Natural hair colors like Henna, etc. only work properly over natural uncolored/ bleached hair. If youtry to use them over chemically treated hair (permed, relaxed, chemically colored) you will end up with a really nasty effect that cannot be fixed with chemical color (ie. you will have to grow out your mouldy-green hair). Read the packaging thouroughly with any hair color...or go see a stylist. You'll be glad you spent the money.